RELATION BETWEEN THE TREE AND THE SHRUB FORM. 67 
in such. Shrubby plants of the heath family are also frequent in 
the natural shrubberies, and some are of large size and quaint form. 
There is a close relationship between the tree form and the shrub 
form. A number of tall shrubs stand on the border-line, so that a 
comparatively slight change of outer circumstances will turn the 
scale. Thus the round-leaved coprosma (Coprosma rotundifolia), one 
of the commonest of the forest-coprosmas, described in Cheeseman’s 
Manual as “a laxly branched shrub 4-12 ft. high,” when growing 
as a constituent of low river-bed forest in Westland is the dominant 
tree, often forming pure “stands,” to use an expressive forestry 
term, and reaching a height of 20 feet or more. But in Stewart 
Island, for example, when growing outside the forest in an exposed 
position, this coprosma is of the dense divaricating growth-form. 
Still more remarkable is the case of the wild-irishman (Discarta 
toumatou). For miles and miles the dark mass of this shrub, a few 
feet high—its form almost that of a divaricating shrub—is an im- 
portant physiognomic feature (fig. 10) of the low tussock-grassland 
plant-formation throughout the South Island. Even when growing 
in the same climate where certain trees flourish it keeps its appa- 
rently fixed and certainly highly-differentiated form. No one seeing 
this shrub in its thousands under the above circumstances would 
ever dream that it could, did circumstances favour, grow into a tree 
20 feet high with a distinct trunk. Yet in the river-valleys of Nelson 
subject to the excessive western rainfall it does become a veritable 
tree, with an actual trunk more than 1 foot through. 
Generally speaking, it is the bushy trees which readily acquire 
the shrub habit —e.g., the broadleaf (Griselinia littoralis) — while 
those shrubs with an altogether different growth-form from trees in 
general, such as the divaricating, do not become trees. 
Dwarfing does not of necessity change a tree into an ordinary 
shrub. The giant rimu (Dacrydiwm cupressinum) when a member 
of the subalpine scrub of Stewart Island is dwarfed to 6 feet in 
stature, but the trunk and tree form persist. Still more remarkable 
is the case of the pahau-tea (Libocedrus Bidwillii), which, an ordinary 
erect tree of the subalpine forest on certain parts of Mount Egmont, 
on the adjacent Pouakai Range, where exposed to the western gales, 
is dwarfed to about 4 feet, and on its leeward side maintains its 
form, resembling a toy tree, but on the windward side its branches 
have disappeared. 
